Telomerase, the ribonucleoprotein known to synthesize and maintain chromosome-protective telomeric DNA, is upregulated in nearly all human tumors. The goal of this research is to understand, and ultimately to exploit therapeutically, this prominent and functionally important hallmark of common human cancers. The level of telomerase upregulation is correlated with tumor grade: highly malignant therapy-resistant tumor cells have higher telomerase levels than lower grade tumors. Increasing evidence suggests that the growth advantage conferred to cancer and other immortal cells by increased telomerase is due not only to its telomere maintenance role but also to function(s) of telomerase separate from this role. Therefore, to fully understand how increased telomerase elevates the survival and proliferative potential of cancer cells, the objective of this proposal i to Aim 1: understand how telomerase interacts with telomeres to promote their maintenance, focusing first on what properties of telomeres signal to cells when telomere function is compromised; Aim 2: define the specific distinct functions of telomerase alternatively spliced isoforms. Some of these telomerase functions involve not only the active reverse transcriptase-competent form of the full telomerase ribonucleoprotein (RNP), but also the abundantly expressed, yet poorly characterized, reverse transcriptase-incompetent b- splice variant of the hTERT protein subunit of the telomerase RNP; Aim 3: dissect the mechanisms of telomere maintenance signaling from the additional mechanisms through which telomerase can act, and understand their impact on important cellular signaling pathways and processes such as growth and apoptosis. Thus, the overall goal of this proposal is to test the hypothesis that the high leve of telomerase characterizing human cancer is selected during cancer progression due to the promotion by telomerase of at least three of the primary hallmarks of cancer: telomere maintenance (immortality); activating invasion and metastasis (cancer stem-like properties; epithelial to mesenchymal transition), and evasion of cell death (anti-apoptotic). Finally, Aim 4 will investigate which of these telomerase/cancer-driver pathway interactions are the critical ones for drugs that kill cancer cells, by testing a novel hypothesis - that moderate, short-term knock- down of telomerase sensitizes cancer stem-like cells to chemotherapy - and will investigate the mechanistic basis of this. The research will use cultured human cancer cells to focus on mechanistic processes within cancer cells, by experimental manipulations and investigations of their molecular consequences. This research is anticipated to generate new approaches for anti-cancer therapies.